The present invention relates to a light beam scanning device, and more particularly to a light beam scanning device in which a synchronizing signal is generated by a PLL circuit from a pulse signal produced by scanning a reference grid plate with a light beam, and an image is formed on the basis of the generated synchronizing signal, the reference grid plate having at its ends slits which are spaced at intervals selected according to the rate of scanning of the light beam for stablizing the synchronizing signal to enable the formation of an accurate image.
In printing and platemaking industries, image scanning recording/reproducing systems are widely used for electrically processing image information on original documents to produce film plates with a view to simplifying the operation process and increasing the image quality.
Such an image scanning recording/reproducing system is basically constructed of an image reading device and an image recording device. In the image reading device, image information on an original document which is fed in an auxiliary scanning direction is scanned by a light sensor in a main scanning direction that is substantially normal to the auxiliary scanning direction, so that the image information can be converted to an electric signal by the light sensor. Then, the image information which has been photoelectrically converted by the image reading device is subject to various processes such as for gradation correction, profile emphasis, and the like dependent on platemaking conditions in the image recording device. Thereafter, the image information thus processed is converted to a light signal represented by a laser beam, which is applied to an image recording medium made of a photosensitive material such a photographic film to record the image information thereon. The image recorded on the image recording medium is then developed by an image developing device, and the developed image recording medium is used as a film plate for printing operation.
In order to accurately reproduce the image by scanning the image recording medium with the laser beam, a synchronizing signal must be generated in synchronism with the scanning cycle of the laser beam. More specifically, in the above image recording device, a reference grid plate is scanned by a synchronizing laser beam in phase with the image recording laser beam, and a pulse signal produced by the reference grid plate is multiplied by a PLL circuit to produce a synchronizing signal. The reference grid plate has a plurality of slits defined therein at equal intervals for generating the pulse signal.
For achieving a smaller, less costly, and more reliable image recording device, it is preferable that a laser diode be employed as a synchronizing laser beam source. The synchronizing laser beam emitted from the laser diode is usually deflected by a galvanometer mirror swinging back and forth to scan the reference grid plate through an f8 lens in the main scanning direction. The f.theta. lens is small and lightweight, and serves to scan the rectilinear reference grid plate at a constant speed with the synchronizing laser beam which has been deflected by the galvanometer mirror.
In the image recording device, the laser beam should preferably be scanned at a higher speed for increasing the image information processing speed. One device for achieving such higher-speed laser beam scanning is a resonant light deflector for angularly moving a mirror at a high speed by utilizing the natural oscillation of an elastic member. Since the resonant light deflector swings at a speed which varies sinusoidally, the synchronizing laser beam is applied via an arc-sine lens to the reference grid plate to keep the scanning speed constant. The arc-sine lens is however disadvantageous in that it is larger in size than the f.theta. lens and causes the laser beam spot to be increased in diameter at the opposite ends of the reference grid plate.
The resonant light deflector may be combined with the f.theta. lens to accomplish higher-speed scanning while keeping the device small in size. Where the resonant light deflector is combined with the f.theta. lens, however, it would be impossible to control the scanning speed of the laser beam on the reference grid plate at a constant level. More specifically, the synchronizing laser beam applied to the reference grid plate is of a higher scanning speed at a central area of the reference grid plate and of a lower scanning speed at opposite ends thereof. Therefore, the frequency of a pulse signal produced from the reference grid plate is higher when the central area of the reference grid plate is scanned by the synchronizing laser beam, and lower when each end of the reference grid plate is scanned by the synchronizing laser beam. As a result, the frequency range of the pulse signal is widened. A PLL circuit for multiplying the pulse signal to produce a synchronizing signal becomes unstable and more susceptible to external noise when the frequency range of the pulse signal is wide. Consequently, no stable synchronizing signal can be produced, and an image cannot accurately be reproduced.